The present invention relates to an image forming apparatus, such as a complex-type digital copying machine that can be provided with option units, an example of which is a sheet discharging device (a finisher).
There is a conventional digital copying machine (an image forming apparatus) that can be provided with option units, an example of which is a sheet discharging device (a finisher). In this type of digital copying machine, the operation of the main body of the digital copying machine is controlled in accordance with the processing speed of the finisher.
The finisher has a function of stapling sheets that have been discharged from the main body of the digital copying machine, and a function of bundling sheets in a designated mode, such as a "sorting/grouping." In addition, the finisher has a function of determining the time required for the sheets from the image forming apparatus to be aligned or stapled. Specifically, in accordance with the types of sheets, the finisher outputs a signal indicating the time interval between the time when the last sheet is discharged and the time when the subsequent sheet is discharged (the time interval will be hereinafter referred to as a sheet processing time), and transmits that signal to the main body of the digital copying machine.
In accordance with the sheet processing time of the signal transmitted from the finisher, the main body of the digital copying machine controls the time intervals at which the sheets are fed. In other words, the main body of the digital copying machine controls the subsequent printing operations in accordance with the sheet processing time.
The main body of the digital copying machine can be provided with an enhanced processing speed, thus enabling a high-speed PPM (Print/Minutes). In actuality, however, the processing speed of the main body is restricted by the sheet processing time of the finisher. For this reason, the digital copying machine provided with the finisher cannot operate at its original high speed, resulting in a low processing speed. Moreover, if the finisher is set in the processing mode in which the sheets are stapled or sorted, the PPM of the main body may be greatly lowered.
If the main body is driven at high speed, with the finisher coupled thereto, the finisher is also required to attain high-speed processing. Suppose there are a high-speed main body and a low-speed main body. In this case, different types of finishers have to be prepared in accordance with the processing speeds of the main bodies. Alternatively, a finisher suitable for use with the high-speed main body has to be coupled to the low-speed main body. Although the high-speed finisher can be coupled to the low-speed main body, this is a waste of cost in many points, particularly in terms of the cost needed for the manufacture and development of the finisher.